The present invention relates to an infrared ray blocking transparent film, for example, a film to be adhered to a windowpane of a building, an automobile, or the like, primarily in order to block infrared rays in sunlight.
Transparent films having infrared ray blocking properties, transparency (transmissivity) to visible light, and properties of reflection or absorbing of infrared light, have been used for controlling the thermal effects of solar radiation. For example, infrared ray blocking transparent films are adhered to windowpanes of buildings, automobiles, and the like so as to reduce heat from direct sunlight being transmitted therethrough. In the summer, the air conditioning load may be reduced by suppressing the elevation of room temperature, and in the winter, the heat insulating efficiency at room temperature is improved. As an additional effect, if the windowpane is broken, the scattering of pieces of glass is prevented. In general, such an infrared ray blocking transparent film is formed in such a way that an infrared ray blocking layer is provided on one surface of a base film composed of a synthetic resin such as PET (polyethylene terephthalate) or the like, a hard coat layer as a surface protective layer is laminated on another surface of the base film, and a separate material composed of paper, film, or the like, is adhered via an adhesive layer on the surface of the infrared ray blocking layer. When the film is to be affixed, the separate material is removed, whereby the adhesive layer may be adhered to a glass plate, etc.
The infrared ray blocking layer has been formed on a base film by coating infrared ray blocking agents such as various kinds of infrared ray absorbers (for example, immonium, aluminum or anthraquinone-type compounds), infrared ray reflecting materials (for example, ZnO, SnO.sub.2, phthalocyan-type pigments, etc.), or the like. However, when the above-mentioned infrared ray blocking agents are used, the layer is a dark brown or a dark blue, whereby transparency of the layer is inferior since visible light transmissivity is lowered to 50% or less. Alternatively, the layer can absorb only infrared rays having longer wavelengths than that ranging from 1000 to 1500 nm, or the layer can absorb only infrared rays having a very small range of wavelengths. Therefore, in order to overcome the above-described conventional defects of the infrared ray blocking agent, indium tin oxide powder (hereinafter referred to as ITO) was developed as a conductive coating material. This ITO powder is coated to form a thin film on a base film by a vacuum deposition method, a sputtering method, a method for coating the coating material for the infrared ray blocking layer by dispersing it into resin, or the like, thereby reflecting infrared rays having wavelengths ranging from 800 to 2500 nm, so that an infrared ray blocking film having high transparency can be provided.
However, in order to provide the above-described ITO powder on the base film by a vacuum deposition method or a sputtering method, a high-vacuum performance apparatus and a high-precision atmosphere controlling system are required, thereby causing problems in that the cost of production is increased and adversely affects mass-productivity, or the like. Therefore, as a method for forming a thin film without these problems, the above-described coating method may be preferably employed. As the resin forming the infrared ray blocking layer by this method, a typical resin for forming a coating film such as a UV-curing resin such as an acrylic-type resin, urethane-type resin, epoxy-type resin, or the like; a thermosetting resin such as melamine resin, polyurethane resin, silicone resin, silicone-modified resin, or the like; and a thermoplastic resin such as an acrylic-type resin, urethane-type resin, or polyester-type resin; or the like, which have high transparency similar to that of a base film, can be chosen and employed suitably. In order to produce the above-described lamination, the following method can generally be employed. A first film in which a hard coat layer is coated on a surface of a base film and an infrared ray blocking layer is coated on a rear surface of the base film and a second film in which an adhesive layer is coated on one surface of a separate material are produced, respectively, and the infrared ray blocking layer of the first film is adhered on the adhesive layer of the second film, whereby both films are laminated to each other.
The infrared ray blocking transparent film is required to have high transparency as well as durability, that is hardness, in the case in which the film is adhered to glass etc. However the above-described thermosetting resin, thermoplastic resin or the like is not cured in a thin film and in the case in which a UV-curing resin is employed, thickness of the film is required to be about 3 .mu.m in order to sufficiently cure, therefore, there was a limit to the improvement of transparency by thinning the film. According to the above-described laminating method, the ITO powder or a pigment included in the resin is exposed on a surface of the infrared ray blocking layer, thereby causing minute unevenness. This unevenness is difficult to bury into the adhesive layer adhered to layers. Therefore, the haze degree thereof increased, whereby high transparency was difficult to obtain. Even if hardness of a surface is obtained by providing a hard coat layer on a film providing an infrared ray blocking layer, with respect to the above resin, there is limit to improvement of hardness of the infrared ray blocking layer, and satisfactory results have not been obtained. In the case in which the film is adhered to a glass, adhesion between each layer becomes insufficient over time due to the effects of sunlight, etc., thereby resulting in peeling off of the layers. In addition, in the case in which the thickness of the infrared ray blocking layer is about 3 .mu.m as described above, a considerable amount of the ITO powder must be used, whereby it was difficult to reduce the cost since the ITO powder is relatively expensive.
Therefore, it is an object of the present invention to provide an infrared ray blocking transparent film which can have higher transparency and which can be produced at lower cost than conventional films, which can improve the hardness of the overall film, and which can exhibit excellent adhesion and superior durability.